The Water Cycle : Feature Articles The Water Cycle By Steve Graham, Claire Parkinson, and Mous Chahine Design by Robert Simmon October 1, 2010 Viewed from space, one of the most striking features of our home planet is the water, in both liquid and frozen forms, that covers approximately 75% of the Earth’s surface. Geologic evidence suggests that large amounts of water have likely flowed on Earth for the past 3.8 billion years—most of its existence. Believed to have initially arrived on the surface through the emissions of ancient volcanoes, water is a vital substance that sets the Earth apart from the rest of the planets in our solar system. In particular, water appears to be a necessary ingredient for the development and nourishment of life. Earth is a water planet: three-quarters of the surface is covered by water, and water-rich clouds fill the sky. (NASA.) Water, Water, Everywhere Water is practically everywhere on Earth. Moreover, it is the only known substance that can naturally exist as a gas, a liquid, and solid within the relatively small range of air temperatures and pressures found at the Earth’s surface.

Water is the only common substance that can exist naturally as a gas, liquid, or solid at the relatively small range of temperatures and pressures found on the Earth’s surface. Sometimes, all three states are even present in the same time and place, such as this wintertime eruption of a geyser in Yellowstone National Park. (Photograph ©2008 haglundc. ) In all, the Earth’s water content is about 1.39 billion cubic kilometers (331 million cubic miles), with the bulk of it, about 96.5%, being in the global oceans. As for the rest, approximately 1.7% is stored in the polar icecaps, glaciers, and permanent snow, and another 1.7% is stored in groundwater, lakes, rivers, streams, and soil. Only a thousandth of 1% of the water on Earth exists as water vapor in the atmosphere. Despite its small amount, this water vapor has a huge influence on the planet. Water vapor is a powerful greenhouse gas, and it is a major driver of the Earth’s weather and climate as it travels around the globe, transporting latent heat with it.

Latent heat is heat obtained by water molecules as they transition from liquid or solid to vapor; the heat is released when the molecules condense from vapor back to liquid or solid form, creating cloud droplets and various forms of precipitation. Water vapor—and with it energy—is carried around the globe by weather systems. This satellite image shows the distribution of water vapor over Africa and the Atlantic Ocean. White areas have high concentrations of water vapor, while dark regions are relatively dry. The brightest white areas are towering thunderclouds. The image was acquired on the morning of September 2, 2010 by SEVIRI aboard METEOSAT-9. [Watch this animation (23 MB QuickTime) of similar data to see the movement of water vapor over time.] (Image ©2010 EUMETSAT. ) Estimate of Global Water Distribution Volume (1000 km3) The Amazing Properties of Water : nrich.maths.org Can you fill in the mixed up numbers in this dilution calculation? The Amazing Properties of Water Stage: 4 and 5 Challenge Level: Why do people say that water is necessary for life? Why have we never found any living organism that can flourish in a completely dry environment?

How is it that something odourless, colourless, tasteless and relatively unreactive makes up 60% of our body mass? Why could another liquid not be used? Why water? Water has several properties that make it unique amongst compounds and make it possible for all forms of known life to function. It is the only natural substance found in all three physical states at the temperatures that naturally occur on Earth. This means that we see water as a liquid, in rivers and seas, a solid, as snow and ice, and as a gas, as clouds or steam. If this was not the case the water cycle would be completely different (what if water didn't evaporate in the sun?); snow and ice might not exist (and thousands of species would now be homeless) and weather would be very different (would it still rain?). Can you think of anything else that would be different? Do you think we could still live if water was only a liquid at the temperatures that occur naturally on Earth? What about only a gas/solid? Ice is less dense than water This just means that ice floats on water and that lakes freeze from the top down to the bottom.

This is clearly important for animals that live on ice, as their habitats would be greatly reduced or not exist at all if ice sank. Similarly, fish and other pond-life would be affected if lakes and ponds froze from the bottom upwards - the layer of frozen water at the top of the pond provides some insulation and prevents the rest of the water getting cold as quickly. It does this by making a barrier between the cold air and the water below. This means that large bodies of water don't get cold deeper down as fast as they might if ice sank and helps wildlife survive in ponds over winter. In fact, water gets more dense at it is cooled until it reaches $4^\circ$ C (which you can see on the graph), after which it gets less dense again. Most substances get progressively more dense as they are cooled. The graph below shows how density of water changes with temperature. Water has a very high melting and boiling point compared to other similar molecules This is what means it is seen as a solid, a liquid and a gas on Earth. If water was not a liquid at most of the temperatures we see on Earth the seas would all be ice, there would be no rain, nothing for plants to collect and animals to drink.

Even our cells are filled with liquid water, which would not be possible either. It is called the 'universal solvent' because it is capable of dissolving so many substances. The water in our bodies is mostly contained in our cells, where it gives them a clear shape as well as having billions of useful molecules dissolved in it. Our cells need to be filled with water to work properly because the enzymes inside them only work in solution. Water is also the means by which transport occurs in our bodies' blood is mostly water and has hormones and gasses dissolved in it as well as toxins such as urea, which are removed from the body with yet more water. Transport in water also occurs on a much smaller scale: when something is dissolved it can move around in the solvent. For example, if an enzyme needs calcium ions to be activated and start working it will meet these as they move about in the solution inside a cell. The higher the concentration of calcium in the cell, the more likely the enzyme will meet an ion sooner. You may also have learnt about diffusion and osmosis, which are key concepts in understanding how cells function and rely on the presence of water Water has a high specific heat capacity.

While this idea sounds a bit intimidating it is actually very simple - all it means is that it takes a lot of energy to make water a little bit warmer. This is why on a hot day the sand on a beach can be too hot to walk on but the sea still feels cool; the energy from the sun is enough to heat the sand a lot but the water only a little. This has some very important implications, especially for organisms that live in water. Seas, lakes and rivers maintain a much more constant temperature than air, which means that animals can live in water all year round without having to adapt to large temperature changes (What do you think would happen if water cooled more easily? What problems might this pose to organisms living in a small pond?). This also means that our body temperature is reasonably difficult to change quickly and hence makes our brain's job of maintaining a constant body temperature much easier (What might happen if our body temperature changed quickly and easily?). Surface tension You might have noticed that if you put a needle on the surface of a bowl of water it floats but if you throw it in it sinks. This is because the needle is denser than water so wants to sink but is held up by the surface tension of the water.

Surface tension is a property that means the surface of water does not want to break - it is 'sticky'. You can also see this when you fill a cup to the brim, as the surface of the water will hold together and resist spilling. The high surface tension of water is the reason that some flies can land on its surface without sinking. This concept is also very closely related to another, the idea of capillary action. Capillary action Similarly to surface tension, this revolves around the idea that molecules of water 'stick' together. If you put a very fine tube into a beaker of water you will see that some of the water travels a short way up the tube; this is called capillary action and is caused by the water clinging to the sides of the tube and to other molecules of water, pulling them up the tube with it. Eventually the weight of the water being pulled is too great to be supported and the water stops moving, having reached an equilibrium.

This same principle allows plants to draw water up from the ground; as water molecules travel up the stem more water molecules stick to them and more stick to those and so on until a column of water is being transported in the xylem (to find out more about this, look up 'transpiration'). But why? All of these properties of water are critical for life as we know it but why does water have them? Does no other substance also do these things? What is it about water that makes it different? To understand the answers to these questions you have to study the molecular structure of water. The chemical formula for water is H2O, meaning that it contains two hydrogen atoms covalently bonded to one oxygen atom, like this: The 95.84pm shows the length of the bond between the oxygen and the hydrogen; one pm is short for a picometre, which is one trillionth of a metre. The angle shown is the acute angle between the two hydrogen nuclei (do you have any ideas about why it isn't $180^\circ$ ? Why should it be $180^\circ$ ?) This diagram represents the bonds as lines, which makes it look like they are the same at every point ?

this is not the case. Because oxygen is more electronegative than hydrogen it attracts the electrons in the O-H bond towards itself. If the thickness of the line represents electron density (the amount of time electrons spend in that part of the bond), the O-H bond actually looks more like this: PICTURE showing polarisation of bond As you can see the electrons spend most of their time nearer to the oxygen nucleus. This means that the oxygen is slightly negatively charged and that the hydrogens are slightly positively charged. This is called polarity and is the key concept behind the properties of water PICTURE TO BE RE SIZED - This means that weak ionic interactions can occur between neighbouring water molecules as the slightly positive hydrogen from one molecule is attracted to the slightly negative oxygen on another. These are called hydrogen bonds. This is why water 'sticks' to itself, giving it high surface tension and the ability to move by capillary action. A great deal of energy is required to break down the hydrogen bonds, which is why the melting and boiling points of water are high and why it has a high specific heat capacity A substance will dissolve in water if it is polar or ionic. This is because it dissolves by forming hydrogen bonds with the water.

Oil is not polar, which is why it will not dissolve. Salt, NaCl, does dissolve because it is ionic. Can you think of some other substances that are soluble in water? How do they form hydrogen bonds with it? Can you draw a similar picture to the one below showing why glucose is soluble? Is it more or less soluble in water than salt? Why do you think this is? A diagram of what happens to NaCl in solution: This is an image of the chemical structure of glucose. There is a C (carbon atom) at each vertex - this is a common way of drawing organic molecules. NOVA - Official Website | Life's Little Essential NOVA Inquiry: AN OCCASIONAL COLUMN Life's Little Essential Everybody knows liquid water is necessary for life, at least as we know it. But just why exactly?

By Peter Tyson Posted 01.04.04 NOVA A friend of mine once had a poster on his office wall that asked at the top in big letters "WHY IS THE SKY BLUE?" I first saw the poster from a distance, and my initial reaction was to snicker slightly, thinking "Everybody knows why the sky is blue." The rest of the poster proved to be the perfect rejoinder, for beneath that simple question lay row upon row of complex equations, originally published by Albert Einstein in 1911, that described in mathematical terms precisely why the sky is blue. When I looked beneath the surface of it, the question that opens this article elicited a similar effect: I was surprised by how much I didn't know about why water is thought necessary for life. Once I learned the particulars, it became clear why planetary scientists on the lookout for life on Mars and elsewhere in the solar system are on the lookout for water. Planetary scientists are quick to stress that it's not just water that's indispensable for life, but liquid water. The distinction is key. Enlarge Photo credit: © Michael Utech/iStockphoto Of the essence So why is liquid water the sine qua non of life as we know it?

Liquid water may sound redundant, but planetary scientists insist on using the qualifier, for solid or vaporous water won't do. The biochemical reactions that sustain life need a fluid in order to operate. In a liquid, molecules can dissolve and chemical reactions occur. And because a liquid is always in flux, it effectively conveys vital substances like metabolites and nutrients from one place to another, whether it's around a cell, an organism, an ecosystem, or a planet. Getting molecules where they need to go is difficult within a solid and all too easy within a gas—vapor-based life would go all to pieces. And why is water the best liquid to do the job? For one thing, it dissolves just about anything. "Water is probably the best solvent in the universe," says Jeffrey Bada, a planetary scientist at the Scripps Institution of Oceanography in La Jolla, Calif. "Everything is soluble in water to some degree." Even gold is somewhat soluble in seawater. (Before you get any ideas about extracting gold from the oceans, I should add that, according to Bada, the value of dissolved gold in a metric ton of seawater comes to about $0.0000004).

Water plays another key role in the biochemistry of life: bending enzymes. Enzymes are proteins that catalyze chemical reactions, making them occur much faster than they otherwise would. To do their handiwork, enzymes must take on a specific three-dimensional shape. Never mind how, but it is water molecules that facilitate this. Ice, ocean, cloud: Water is the only chemical compound on our planet's surface that comes naturally in all three physical states. Enlarge Photo credit: © Peter Tyson Black sheep of the liquids Water's ability to so successfully further the processes of life has a lot to do with just how unusual a fluid it is. Not long ago, if I had to guess, I would have said that water is about as normal a liquid as they come. In fact, despite its ubiquity and molecular simplicity, H2O is abnormal in the extreme. Liquid water is still the Holy Grail for planetary scientists. For starters, while other substances form liquids, precious few do so under the conditions of temperature and pressure that prevail on our planet's surface. In fact, next to mercury and liquid ammonia, water is our only naturally occurring inorganic liquid, the only one not arising from organic growth.

It is also the only chemical compound that occurs naturally on Earth's surface in all three physical states: solid, liquid, and gas. Good thing, otherwise the hydrological cycle that most living things rely on to ferry water from the oceans to the land and back again would not exist. As science journalist Philip Ball writes in his informative book Life's Matrix: A Biography of Water , "This cycle of evaporation and condensation has come to seem so perfectly natural that we never think to remark on why no other substances display such transformations." At sea level, water boils at 212°F, but thousands of feet down in the ocean, the pressure can keep water liquid at over 650°F, as at this hydrothermal vent. Enlarge Photo credit: © Ralph White/CORBIS Compared to most other liquids, water also has an extremely large liquid range. Pure water freezes at 0°C (32°F) and boils at 100°C (212°F). Add salt and you can lower the freezing temperature; natural brines are known with freezing points below -50°F. Add pressure and you can raise the boiling temperature; deep-sea vent waters can reach over 650°F.

Water also has one of the highest specific heats of any substance known, meaning it takes a lot of energy to raise the temperature of water even a few degrees. Water's broad liquid range and high heat capacity are good things, too. They mean that temperatures on the Earth's surface, which is more than two-thirds water, can undergo extreme variations—between night and day, say, or between seasons—without its water freezing or boiling away, events that would throw a big wrench into life as we know it. As it is, the oceans serve as a powerful moderating influence on the world's climate. Liquid water has yet another unusual property that means the difference between life and essentially no life in cold regions of the planet. Unlike most other liquids when they freeze, water expands and becomes less dense. Most other frozen liquids are denser than their melted selves and thus sink. If it sank, ice, being unable to melt because of the insulating layer of water above it, would slowly fill up lakes and oceans in cold climates, making sea life in those parts of the world a challenging prospect. Saturn's moon Titan has organic chemicals like methane and ethane in its thick atmosphere, leading some scientists to speculate on whether hydrocarbon-based life might exist there.

Enlarge Photo credit: © NASA, Voyager 2 Waterless life Could life as we don't know it have gotten a start without water? Some planetary scientists have suggested that on certain very cold planetary bodies liquid ammonia might serve in place of water to incubate life. But even though it's the most common non-aqueous solvent, liquid ammonia would seem to have several other things going against it as a medium for life. Its liquid range is small, only about 30 degrees. Also, when it freezes, it sinks, and we know what that would do. Some have suggested that oceans of methane or other hydrocarbons on places like Saturn's moon Titan could also serve the purpose. But, again, we're talking temperatures so low that chemical reactions as we know them could only proceed at a glacial pace. "At minus 150 degrees," says Bada, "most of the reactions that we think about in terms of being important in the origin of life probably wouldn't take place over the entire age of the solar system." Moreover, compounds like amino acids and DNA would not be soluble in these other liquids. "They would just be globs of gunk," Bada says.

For these and other reasons, liquid water is still the Holy Grail for planetary scientists, who, based on what they know today, consider it likely that liquid water is essential to all life, terrestrial and extra-. Says Neil de Grasse Tyson, an astrophysicist and director of the Hayden Planetarium at the American Museum of Natural History, "Given that life on Earth is so dependent on water, and given that water is so prevalent in the universe, we don't feel that we're going out on a limb to say that life would require liquid water." Just as a blue sky requires blue light to scatter far more than all the other colors in the visible light spectrum—which, of course, is why the sky is blue. Well, that's the simple answer anyway. Peter Tyson is editor in chief of NOVA Online. Related Links States of Matter: Facts (Science Trek: Idaho Public Television) For a better understanding, take a look at these animations of the behavior of solids, liquids and gases . Change of State Matter can move from one state to another, but can still be the same substance.

A change of state, also called a phase change , is a physical change from one state of matter to another, for example, from solid to liquid or from liquid to gas. How does matter move from one phase to another? If the motion of the atoms is altered by pressure or temperature, the state can change too. By lowering the temperature of water, it can freeze into a solid. By heating water, it can become steam which is a gas. Whether solid, liquid or gas &dash; water is still water. Pressure can change matter from one state to another. Deep in the earth solids turn to liquids because the heavy weight of layers and layers of the earth push down on the solids causing them to turn to liquid magma . This is just one example of how pressure can change matter too. Other matter changes too, but often only exists in two states or requires the help of humans and technology to move through all three phases. Water is the only matter on earth that can be found naturally in all three - solid, liquid and a gas. More About Atoms An atom is made up of a nucleus of neutrons and protons &dash; even smaller particles than an atom. They are called subatomic particles . Electrons circle around this nucleus.

Atoms tend to have the same number of electrons as the number of protons in the nucleus. This number is very significant. Each kind of atom has a different number of protons. For example: oxygen has 8 protons. That means it also has 8 electrons. The 8 is known as its atomic number. But gold has 79 protons and 79 electrons. If you wanted to know how many protons and electrons are found in any given atom, the Periodic Table of the Elements is the place to find out. Elements The table only tells us about the atoms of the 100 or so elements found on the earth. An element cannot be broken down into smaller components . Gold is not made of anything &dash; except the element gold. The smallest possible piece of gold is just gold. If it were any smaller it would be the subatomic particles &dash; protons, neutrons and electrons. Of these elements, some are solids like gold or silver. Others are gases like oxygen and hydrogen, and still others can be liquid like mercury. Of course this is based on room temperature. If an element is heated or cooled, or if enough pressure is applied to the element that could change its state of matter.

Compounds I bet you can't find salt on the Periodic Table of the Elements. That's because salt is actually made of two elements; sodium and chloride. You can find both of those on the table. The sodium and chloride atoms of salt are linked together much like magnets can link together. We call this a compound . Water is a compound made of oxygen and hydrogen. Sugar &dash; the white stuff you put into cookies &dash; is made of carbon, hydrogen and oxygen. Lots of the substances around us are compounds. From vinegar to household bleach to nail polish remover to baking soda to aspirin &dash; we use compounds all around our homes. Properties All matter has qualities about it that describe it scientifically. We call these properties . They can explain the physical or the chemical qualities of a matter. Some of the ways that we describe matter are by its color, its shininess, and its state at room temperature or its odor. Mass, temperature at which it changes states, electric conductivity and flexibility are also properties of matter. These are different for each type of matter and can help scientists identify a matter. Plasma We often talk about the three states of matter; solid, liquid and gas.

Most of the matter that we use is in one of those three forms. But there is another that we see and use and that we would have a hard time living without &dash; plasma . It is often called the fourth state of matter. Plasma is electrically charged, does not hold its shape, has a huge amount of energy and is very difficult state to manipulate without a laboratory. Plasma can be found here on the earth in flames, lightning, and the polar auroras. The sun, the stars, and some other space events and objects are also made of plasma matter. More States of Matter? Did you know that there are more phases of matter? We aren't as familiar with them nor do we see them every day. Some exist only in theory, others can be reproduced in laboratories, some are so new that scientists are still figuring out the details and others might exist, but have not yet been found in nature. They include: quark-gluon plasmas, Bose-Einstein condensates, fermionic condensates, strange matter, liquid crystals, quantum spin liquid (QSL), superfluids, supersolids and the paramagnetic and ferromagnetic phases of magnetic materials.

New theories and discoveries are happening all of the time, so you'll have to keep studying your physical science to learn more about these states of matter. What is meant by room temperature? State of matter of an element or atom is based on its behavior at room temperature. But what exactly is room temperature? Room temperature refers to air temperature not being specifically heated or cooled. Usually between 20 to 25°C (68 to 77°F). While the actual temperature could be hotter or cooler than these figures, it simply means that the matter has not been placed in an oven or a freezer, but was left out in the room to attain the temperature of the rest of the room. If an ice cube was left out in the room for a period of time, it would melt and become liquid water. Water is a liquid at room temperature. A diamond, on the other hand, is a solid at room temperature and will not change state no matter how long it sits there. The Chemistry of Water | APEC Water THE CHEMISTRY OF WATER Tweet Water is a phenomenal substance. Its presence brings life and growth to all manner of organisms, and consequently its absence brings the opposite.

Covering 80% of the Earth's surface, it is the only substance that occurs naturally on the planet in all three states; solid, liquid, and gas. For being such an important substance, it is remarkably simple in its makeup. Water is composed of two parts hydrogen and one part oxygen, making the chemical symbol H2O. Properties of Water Water exists naturally on the planet in all three states. When the temperature of the water is below freezing (32 degrees Fahrenheit), it is a solid. Between the freezing point and the boiling point, it is a liquid. And above the boiling point (212 degrees Fahrenheit), it is a gas. Aside from the three states, water has a number of unique properties. Most liquids get smaller as they get colder, but not water. Water will stop getting smaller at about 39 degrees Fahrenheit, and then will reverse and start to expand as it gets even colder. Another property is referred to as cohesion, which means that water is attracted to other water. Water Properties – A good basic overview of the properties of water, with pictures and diagrams that increase understanding. Physical Properties of Water – This information from the University of Minnesota expands a bit on some information from the first link and offers some new facts.

Water Experiments – Although this is designed for middle school and junior high kids, these experiments are easy to accomplish and will illustrate the properties. Properties Compared – Here you can learn about the properties of water in comparison to salts, oils and detergents. Water Cycle – A good summary, to include a very descriptive picture, on the water cycle on the planet. Water Molecules Water is composed of two hydrogen atoms and one oxygen atom. They are attracted to each other due to their electrostatic attraction. All that means is the hydrogen atom is positively charged, while the oxygen atom is negatively charged. Opposites attract, and water is no different. Water is attracted to other water because of this charge attraction, in the same manner that the atoms are combined. The hydrogen of one water molecule will bond to the oxygen atom of another molecule. Sticky Water – An explanation and experiment on the surface tension property of water. Model of a Water Molecule – Explore a three dimensional model of a water molecule, and gain some insight into its construction. Structure Means Foundation – Good information and pictures about the structure and makeup of water molecules. Water Polarity – More information on the charges of the particles that make water and how they attract each other.

Water's Chemical Properties – Learn more about how gravity can change water, as well as how the structure of water changes with the temperature. States of Water – What happens to the molecules as water changes between the three states? Do they remain the same? Find out here. Solubility The solubility of a given liquid simply means the amount of a solid that can be dissolved into the liquid. Solubility depends on a few factors such as the liquid being used, the ambient temperature and the atmospheric pressure. At some point, only so much of a solid can be dissolved into the water before it reaches equilibrium, which means that no more of the solid can be dissolved. Think of this as pouring sugar into a cup of coffee. Only a certain amount of sugar can be dissolved into the coffee before no more can be added. Solubility – A chemically-heavy page, here is a lot of information on solubility, to include rules and more on equilibrium. Vitamin Solubility – More information on the solubility of solids, but given from a molecular standpoint. Soluble or Insoluble – A short listing on the types of compounds that are or are not soluble in water.

Determine Solubility – A chemistry laboratory manual that offers steps on determining the solubility of a substance in water. Solubility and Temperature – Learn more about how temperature affects the solubility characteristics of water. Water (Liquid) When water is in its liquid form, it is the easiest to see. It covers the majority of the planet, after all. Some specific variables have to be just right for water to change states, and those variables have to remain within a certain range for water to remain in whatever state it is in. For example, water must be between 32 and 212 degrees Fahrenheit to remain as a liquid. The primary route for liquid water to return to the Earth's surface is through precipitation. Condensation is the way that water vapor will turn back into liquid water. This causes clouds to form, which in turn cause precipitation, bringing water back to us. Condensation – A wealth of information on the role of condensation in the water cycle. Water and Mars – Great information on why water cannot exist on Mars, which illustrates the properties that allow water to exist in certain states. Why is Water Blue? - A molecular investigation of why liquid water appears to be of a blue color.

Liquid Water Crystals – Questions and answers about liquid water and its crystalline structure. Liquid Mysteries – One would assume that scientists would know everything there is to know about liquid water, but there are some startling questions to be answered. Liquid Water Below Freezing – Water freezes at 32 degrees Fahrenheit, but somehow, scientists have found a way to keep it liquid even below freezing. Ice Quite simply, ice is the frozen state of water. As water freezes, it expands and traps air molecules inside of it. Actually, it is the only known substance (non-metallic) that will expand in this manner. This is why ice will float in a glass of water. It does not necessarily need to be in a liquid state in order to become ice. Water vapor can form ice crystals, as in the form of frost. Ice can be found in many forms in the environment, from snow and hail to icebergs and icicles. Physical Properties of Ice – An explanation (with diagrams) on the structure of frozen water. Freezing Water Expansion – Here you can learn more about how and why water expands during freezing. Density of Ice – Molecular reasoning and more diagrams on the density of water and ice.

Salt and Ice Water – Questions and explanations on the interaction of salt and water and how it affects the freezing process. Dry Ice – Explore the difference between regular ice and dry ice. Ice and the Water Cycle – The role ice plays in the three stage water cycle, and why it is vital to the whole operation. Ancient Ice Cores – Come ice has been around for thousands of years, and some things that are stilled trapped in the ice may hold clues to our future. Water Vapor Water exists as a vapor during its gas state. This can occur through a couple of ways, but mainly at temperatures above 212 degrees Fahrenheit. Above this temperature, water boils and will turn into visible vapor in the form of steam. Water can also exist below this temperature, such as when you are able to see your breath on a cold day. Another form of water turning into vapor is through evaporation, which is the absorption of liquid water into the air. An example of this would be a wet surface allowing to dry. That water didn't disappear, it simply evaporated into the surrounding air. Water Vapor Myths – A brief tutorial, heavy on the text, that debunks some commonly held beliefs.

Vapor and the Water Cycle – The role vapor plays in the three stage water cycle; evaporation drives the water cycle. Water Vapor in Space – Water is critical, in all three stages, for human life to exist. Learn more about its existence in space. The Climate – Learn more about the role that water vapor plays in regulating the climate on the planet. Clouds – Some brief information on the formation of clouds from vapor. Gases in the Atmosphere – Water is just one of the gases found in our atmosphere, but where is it all? USATODAY.com 05/20/2005 - Updated 11:05 AM ET Water's three phases are a key to the weather Source: USA TODAY research by Chad Palmer By Jack Williams, USATODAY.com Water is so common that most of us, most of the time pay little attention to it even though without it life as we know it would not exist. Everyone is aware of water when it falls from the sky as rain, snow, or ice. Its importance to weather, however, goes far beyond the various forms that fall on us.

Water, as it turns out, is one of the main sources of the energy needed to run the Earth's weather machine. To see why, let's look at some basic chemistry and physics. All substances, including water of course, can exist in three phases: Solid, liquid, and gas. Without getting into too much detail, the average speed of molecular motion determines a substance's phase. Temperature, in turn, is a measure of the average speed of molecular motion. But, difference substances change from one phase to another at vastly different temperatures. Nitrogen, for instance, becomes solid (freezes) at -346 degrees F. Since the coldest temperature ever recorded on Earth is -129 degrees F (at the Russian Vostok Station in Antarctica), nitrogen never naturally freezes on Earth. Metals, on the other hand, remain sold at temperatures above 300 degrees F, much hotter than any life yet found can survive. Water is special because it's the only substance that can exist in all three phases at Earth's ordinary temperatures, and it's common to have all three phases together at the same time.

In other words, water's gas form - water vapor, the liquid form (the water we buy in expensive plastic bottles to drink), and the solid form, ice are often found together in clouds. The water molecules in ice are locked into a hexagonal crystal and do not move freely. Liquid water molecules are moving fast enough to break free from the crystal structure, but they are still attached to each other. This allows liquid water to assume the shape of its container. When water is a gas - vapor - the molecules are free to move about. To understand the weather, you need to understand what happens when water changes its state. These changes are: Evaporation: From liquid to gas (water vapor). UCSB Science Line UCSB Science Line I heard Mercury and water are the only substances to go through all 3 phases of matter. Is that true, and if not what are the other substances? Answer 1: Mercury and water are not the only substances capable of existing in three distinct states of matter. In fact, all of the elements, of which mercury is one, may exist in solid, liquid, or gas forms. Additionally, many substances exhibit more than one solid form, often with very different properties.

For example, both graphite and diamond are composed of carbon, but the arrange of carbon atoms within the solid is different. For simplicity, consider pure substances such as water. The stable form of matter, or phase, of a single component system depends on the temperature and pressure of the system. We are typically used to atmospheric pressure. Therefore, we expect water to be a liquid at room temperature, to freeze to a solid below 32 degrees Fahrenheit (0 deg Celsius), and to boil into the gas phase above 212 deg F (100 deg C). However, if you placed a glass of water inside a vacuum and reduced the pressure to one tenth of the normal atmospheric pressure, the water would boil even in a nicely air conditioned room. The same is true for other substances, although the temperatures and pressures at which a given phase is stable may be so extreme that for all practical purposes you'll never encounter it. For example, the gas phase of copper is stable at atmospheric pressure only at temperatures above 4643 deg F (2562 deg C); however, if you wanted to obtain the gas phase near room temperature, you would have to reduce the pressure to less than one billionth of atmospheric pressure.

Answer 2: This is not true; in fact all of the elements except for helium go through all three phases of matter (solid, liquid, and gas) as the temperature is varied. That is to say, at low enough temperatures, all the elements (except Helium) are solids. As the temperature is increased they will melt and become liquids. Each element melts at a different temperature, called it's "melting point". Hydrogen melts at -259 degrees Celsius. Mercury melts at -39 degrees Celsius. Iron melts at 1,535 degrees Celsius. As the temperature is increased still further, each element will boil and become a gas. Each element boils at a different temperature, called the "boiling point". For Hydrogen, the boiling point is -253 degrees Celsius; for mercury it is 357 degrees Celsius, and for iron it is 2,750 degrees Celsius. From these numbers you can see that at room temperature (and indeed for nearly any temperature found on Earth) hydrogen is a gas, mercury is a liquid, and iron is a solid. This discussion also holds true for small molecules, like water (H2O), carbon dioxide (CO2), etc.

Water is the only substance where all three states can be readily observed in everyday life: solid water as ice, liquid water in a water fountain, and gaseous water as steam. Carbon dioxide is a good example of a substance which can be a liquid, but only when it is under pressure. For example, at room temperature unpressurized carbon dioxide is a gas, but if it is pressurized to 800 pounds per square inch (as it often is in steel bottles), it becomes a liquid. Helium (an element) is unique in that it will not freeze no matter how cold it is made unless it is simultaneously very cold (colder than -270 degrees C) and under a great deal of pressure (about 400 pounds per square inch). Thus you can see that all the elements and all the simple molecules can be made to go through all three phases if you can control the temperature and the pressure they experience. It might seem that this would mean that any substance could be made to be solid, liquid or gas, but this is not quite true.

For example wood, which is made out of very large, complicated molecules containing mostly carbon, hydrogen, oxygen, and nitrogen, does not boil - i.e., when you heat it it does not melt into liquid wood and then boil into gaseous wood. Instead as the wood becomes hot (about 300 degrees Celsius), the big molecules break up into smaller molecules like carbon dioxide. Carbon dioxide is a gas at 300 degrees Celsius, so it floats away. This process is called "burning", and is the result of the big molecules of the solid wood breaking up into small molecules like Carbon Dioxide, which then float away as a gas. If you are interested, there is a table of all the melting and boiling points of all the elements at Module Overview Tropospheric Ozone Global Climate Change Before starting the activities, you are encouraged to read the Overview below and the background material provided in each section. It will give you a broad introduction to the science content found in this module. Introduction It would be among the most profound discoveries in the history of our species if we could detect life on other planets circling other stars in the galaxy.

Whether the life turns out to be intelligent, technological, or simply microbial, such a discovery would provide evidence that we are not alone, that life (in one form or another) is widespread in a universe that suddenly seems less stark and empty. How might we detect such distant life? Not likely with fleets of starships. The energies required for star travel (barring a science-fiction style warp drive) are simply too great and travel times too long. Instead, sophisticated ground and space-based telescopes are being used to detect planets of nearby star systems. How planets block the light of their star as detected from Earth can reveal the chemical composition of the planet's atmosphere. Some scientists believe that the chemical composition of the planet's atmosphere will provide evidence of the presence of life. How is this possible? How can simple gases tell us about the presence or absence of life as we know it? To answer that, we must look at our own planet. Earth has a radius of some 6400 km. Ninety-nine percent of the earth's atmosphere is contained within a layer approximately 50 km thick.

Life on earth inhabits a layer no more than 9 km thick, extending from a bare few kilometers above sea level (airborne organisms and life on mountains) to a few kilometers below (deep ocean basin creatures and subterranean microbial communities). The biosphere is the portion of earth in which all known life forms exist. It occupies a thin layer of air (atmosphere), water (hydrosphere), and land (lithosphere). Cycles Just as we earthly organisms require a source of energy, water, and the chemical components of our bodies, so too does the entire global biosphere. These services are provided to the biosphere by global energy and chemical cycles. Most people are familiar with the concepts of cyclesthat certain substances move endlessly throughout the earth's biosphere, hydrosphere, atmosphere, and lithosphere, existing in different forms and being used by different organisms at different times, but always moving, always circulating. Most of your students are probably familiar with the water cycle. Water, in its different forms, cycles continuously through the lithosphere, hydrosphere, atmosphere, and biosphere. Water evaporates into the atmosphere from the land and the sea.

Plants and animals use and reuse water and release water vapor into the air. Once in the air, water vapor circulates and can condense to form clouds and precipitation, which fall back to earth. At one time or another, all of the water molecules on earth have been in an ocean, a river, a plant, an animal, a cloud, a raindrop, a snowflake, or a glacier! In addition to water, many other substances such as nitrogen, oxygen, and carbon cycle through the earth and atmosphere. These cycles are important to individual animals and plants and even to entire ecosystems. But we're less familiar with the notion that these cycles fundamentally influence the planet as a whole, dramatically and unmistakably altering the earth's atmosphere. When you think about it, this influence only makes sense. The atmosphere is the greatest, fastest, and most reliable global transport system we have. Inject almost any stable gaseous material into the atmosphere and before long it is spread worldwide. For example, the graphic below shows how the smoke and ash plume from the eruption of Mt. Spurr in Alaska spread and moved over a 4-day period, as detected by satellite imagery.

During that time, the original plume was carried rapidly eastward from Alaska and spread over an area many times its original size. Because cycles require the movement of substances, what better conveyor belt to use than the atmosphere? To explain what we mean, let's look at several important components of the earth's atmosphere and see how they are influenced by these cycles. Nitrogen comprises the bulk of the atmosphere (approximately 78%). Nitrogen cycles slowly through the earth's system. To most of the biosphere, nitrogen in the atmosphere is like the ocean to a thirsty personamazingly abundant but not quite in the right chemical form. A molecule of nitrogen gas is made up of 2 atoms very tightly bound together. It takes tremendous amounts of energy, such as produced by lightning or fires, to break the bond. Amazingly, an assortment of bacterial species that specialize in taking nitrogen from the air can also convert nitrogen into different usable forms. These bacteria also release nitrogen from organic material back into the atmosphere. Nitrogen is the one element found almost entirely in the atmospherethere's very little on land or in the sea. Nitrogen is essential to life, a key element in proteins and DNA.

Oxygen is found in the atmosphere at a stable concentration of approximately 21%. Because it is a very reactive element, it can quickly combine with other elements and disappear from the atmosphere. Yet it persists, and in high concentration. It is the cycling of oxygen through photosynthesis and respiration that accounts for its presence and stability. A world without cycles, without life, would retain little if any oxygen in its atmosphere. Oxygen does more, however, than simply move between organisms and the atmosphere. Some of the atmospheric oxygen ( ) finds itself lofted high into the upper reaches of the atmosphere called the stratosphere. There, in a series of reactions powered by solar radiation, it is converted into a new compound, ozone ( ). The presence of stratospheric ozone benefits us creatures of the biosphere tremendously! Ozone serves to absorb biologically damaging ultraviolet (UV) radiation from the sun. Without an ozone layer, the earth's surface would be bathed in high intensity UV radiation; with an ozone layer, the amount of UV radiation received at the surface is vanishingly small. Eventually, each ozone molecule will come apart, reform , and may either be carried elsewhere in the atmosphere or may take part again in the ozone-forming reactions.

Over hundreds of millions of years (at least), these ozone-forming and destroying reactions have generally been in equilibrium, forming a balanced - cycle within the greater cycle of atmospheric oxygen. The Atmosphere in Dynamic Equilibrium Oxygen and nitrogen are not the only elements that cycle through the biosphere. Most elements critical to life constantly cycle. This is why earth's atmosphere can be described as being in a state of dynamic equilibrium. Things are constantly moving and changingsubstances enter and leave the atmosphere, forming different compounds at different times and in different places. On earth, the dynamic equilibrium changes with season. For example, in the spring and summer, growing plants take carbon from the soil and atmosphere. In the fall and winter, plants release carbon to the soil and the atmosphere. Because most of the plant life on earth is found in the Northern Hemisphere, there are global seasonal changes of carbon dioxide in the atmosphere. An atmosphere in static equilibrium (like that of Mars) indicates a dead world. All the reactions have taken place and the elements have found their most stable chemical form. The atmosphere of a living planet, like ours, is quite different. Unstable, interesting, and improbable reactions happen all the time, thanks to cycles!

Concluding Thoughts We suggest you use this module to encourage your students to explore the cycles that make up their world. We also encourage you to challenge your students to look at the atmosphere and its influence on individuals, ecosystems, and the planetary biosphere as it moves, reacts, and changes under the influences of the lithosphere, hydrosphere, and biosphere. Cycles are the recurrent theme in the seven units: Properties of Water Infobox disclaimer and references Water ( H 2 O , HOH) is the most abundant molecule on Earth's surface, composing 70-75% of the Earth's surface as liquid and solid state in addition to being found in the atmosphere as a vapor. It is in dynamic equilibrium between the liquid and vapor states at standard temperature and pressure . At room temperature , it is a nearly colorless , tasteless , and an odorless liquid. Many substances dissolve in water and it is commonly referred to as the universal solvent ; because of this, water in nature and in use is rarely clean, and may have some properties different than those in the laboratory. However, there are many compounds that are essentially, if not completely, insoluble in water.

Water is the only common, pure substance found naturally in all three states of matter ï¿½for other substances, see Chemical properties . Contents See the Category:Forms of water Water can take many forms. The solid state of water is commonly known as ice (while many other forms exist; see amorphous solid water ); the gaseous state is known as water vapor (or steam ), and the common liquid phase is generally taken as simply water. Above a certain critical temperature and pressure (647 K and 22.064 MPa ), water molecules assume a supercritical condition, in which liquid-like clusters float within a vapor-like phase. Heavy water is water in which the hydrogen atoms are replaced by its heavier isotope , deuterium . It is chemically almost identical to normal water. Heavy water is used in the nuclear industry to slow down neutrons . [ edit ] A common substance [ edit ] Water in the Universe Water has been found in interstellar clouds within our galaxy , the Milky Way . It is believed that water exists in abundance in other galaxies too, because its components, hydrogen and oxygen , are among the most abundant elements in the universe .

Interstellar clouds eventually condense into solar nebulae and solar systems , such as ours. The initial water can then be found in comets , planets , dwarf planets , and their satellites . In our solar system, water, in ice form, has been found: on the planets Mercury , Mars , and Neptune , on the dwarf planet Pluto , on satellites of planets, such as Triton and Europa . The liquid form of water is only known to occur on Earth, though strong evidence suggests that it is present just under the surface of Saturn 's moon Enceladus . [ edit ] Water on Earth The water cycle (known scientifically as the hydrologic cycle) refers to the continuous exchange of water within the hydrosphere , between the atmosphere , soil water, surface water , groundwater , and plants . Earth's approximate water volume (the total water supply of the world) is 1,360,000,000 km3 (326,000,000 mi3).

Of this volume: 1,320,000,000 km3 (316,900,000 mi3 or 97.2%) is in the oceans 25,000,000 km3 (6,000,000 mi3 or 1.8%) is in glaciers and icecaps 13,000,000 km3 (3,000,000 mi3 or 0.9%) is groundwater . 250,000 km3 (60,000 mi3 or 0.02%) is fresh water in lakes, inland seas, and rivers. 13,000 km3 (3,100 mi3 or 0.001%) is atmospheric water vapor at any given time. Liquid water is found in bodies of water, such as an ocean , sea , lake , river , stream , canal , or pond . The majority of water on Earth is sea water . Water is also present in the atmosphere in both liquid and vapor phases. It also exists as groundwater in aquifers .

The boiling point of water is directly related to the barometric pressure at a particular point by the formulae: pressure (in. Hg) = 29.921* (1-6.8753*0.000001 * altitude, ft)^5.2559 boiling point = 49.161 * ln(in. Hg) + 44.932 For example, on the top of Mt. Everest water boils at about 68 degrees Celsius, compared to 100 degrees at sea level . Conversely, water deep in the ocean near geothermal vents can reach temperatures of hundreds of degrees and remain liquid. [ edit ] Water in industry Water is also used in many industrial processes and machines, such as the steam turbine and heat exchanger , in addition to its use as a chemical solvent . Discharge of untreated water from industrial uses is pollution . Pollution includes discharged solutes ( chemical pollution ) and discharged coolant water (thermal pollution). Industry requires pure water for many applications and utilizes a variety of purification techniques both in water supply and discharge.

[ edit ] Physics and chemistry of water [ edit ] Density of water and ice For most substances, the solid form of the substance is more dense than the liquid phase ; thus, a block of pure solid substance will sink in a tub of pure liquid substance. But, by contrast, a block of common ice will float in a tub of water because solid water is less dense than liquid water. This is an extremely important characteristic property of water. At room temperature , liquid water becomes denser with lowering temperature, just like other substances. But at 4 ï¿½C, just above freezing, water reaches its maximum density , and as water cools further toward its freezing point, the liquid water, under standard conditions, expands to become less dense. The physical reason for this is related to the crystal structure of ordinary ice , known as hexagonal ice Ih . Water, gallium , bismuth , acetic acid , antimony and silicon are some of the few materials which expand when they freeze; most other materials contract. It should be noted however, that not all forms of ice are less dense than liquid water. For example HDA and VHDA are both more dense than liquid phase pure water.

Thus, the reason that the common form of ice is less dense than water is a bit non-intuitive, and relies heavily on the unusual properties inherent to the hydrogen bond . Generally, water expands when it freezes because of its molecular structure , in tandem with the unusual elasticity of the hydrogen bond and the particular lowest energy hexagonal crystal conformation that it adopts under standard conditions. That is, when water cools, it tries to stack in a crystalline lattice configuration that stretches the rotational and vibrational components of the bond, so that the effect is that each molecule of water is pushed further from each of its neighboring molecules. This effectively reduces the density ρ of water when ice is formed under standard conditions. The importance of this property cannot be overemphasized for its role on the ecosystem of Earth. For example, if water were more dense when frozen, lakes and oceans in a polar environment would eventually freeze solid (from top to bottom). This would happen because frozen ice would settle on the lake and riverbeds, and the necessary warming phenomenon (see below) could not occur in summer, as the warm surface layer would be less dense than the solid frozen layer below. It is a significant feature of nature that this does not occur naturally in the environment.

Nevertheless, the unusual expansion of freezing water (in ordinary natural settings in relevant biological systems), due to the hydrogen bond , from 4 ï¿½C above freezing to the freezing point offers an important advantage for freshwater life in winter. Water chilled at the surface increases in density and sinks, forming convection currents that cool the whole water body, but when the temperature of the lake water reaches 4 ï¿½C, water on the surface decreases in density as it chills further and remains as a surface layer which eventually freezes and forms ice. Since downward convection of colder water is blocked by the density change, any large body of fresh water frozen in winter will have the coldest water near the surface, away from the riverbed or lakebed. This accounts for various little known phenomena of ice characteristics as they relate to ice in lakes and "ice falling out of lakes" as described by early 20th century scientist Horatio D. Craft. The following table gives the density of water in grams per cubic centimeter at various temperatures in degrees Celsius: Temp (ï¿½C) The values below 0 ï¿½C refer to supercooled water.

[ edit ] Density of saltwater and ice The situation in salt water is somewhat different. Ice still floats to keep the oceans from freezing solid (see following paragraph). However, the salt content of oceans both lowers the colligative freezing point by about 2 ï¿½C and lowers the temperature of the density maximum of water to be about at the freezing point. Hence, in ocean water, because of the salt content, the downward convection of colder water is not blocked by an expansion of water as it becomes colder near the freezing point; thus the oceans' cold water near the freezing point continues to sink. For this reason, any creature attempting to survive at the bottom of such cold water as the Arctic Ocean generally lives in water that is 4 ï¿½C colder than the temperature at the bottom of frozen-over fresh water lakes and rivers in winter. As the surface of salt water begins to freeze (at −1.9 ï¿½C for normal salinity seawater, 3.5%) the ice that forms is essentially salt free with a density approximately that of freshwater ice.

This ice floats on the surface and the salt that is "frozen out" adds to the salinity and density of the seawater just below it. This more dense saltwater sinks by convection and the replacing seawater is subject to the same process. This provides essentially freshwater ice at −1.9 ï¿½C on the surface. The increased density of the seawater beneath the forming ice sinks towards the bottom, thus the deep ocean waters should have a minimum temperature of −1.9 ï¿½C also. However the temperature of the deep oceans is about 4 ï¿½C. [ edit ] Compressibility The bulk modulus of water is 2.2ï¿½109 Pa . [2] . The stiffness of nongasses makes them incompressible, or of less compressibility; but as their elements are not near degeneracy pressure , they are not noncompressible. Even in the deep oceans at 4000 m depth where pressures are 4ï¿½107 Pa, there is only a 1.8% change in volume.

[3] [ edit ] Triple point The temperature and pressure at which solid, liquid, and gaseous water coexist in equilibrium is called the triple point of water. This point is used to define the units of temperature (the kelvin and, indirectly, the degree Celsius and even the degree Fahrenheit ). The triple point is at a temperature of 273.16 K (0.01 ï¿½C) by convention, and at a pressure of 611.73 Pa . This pressure is quite low, about 1/166 of the normal sea level barometric pressure of 101,325 Pa. The atmospheric surface pressure on planet Mars is remarkably close to the triple point pressure, and the zero-elevation or "sea level" of Mars is defined by the height at which the atmospheric pressure corresponds to the triple point of water. [ edit ] Mpemba effect The Mpemba effect is the surprising phenomenon whereby hot water can, under certain conditions, freeze sooner than cold water, even though it must pass the lower temperature on the way to freezing. However, this can be explained with evaporation , convection , supercooling , and the insulating effect of frost .

[ edit ] Hot ice Hot ice is the name given to another surprising phenomenon in which water at room temperature can be turned into ice that remains at room temperature by supplying an electric field on the order of 106 volts per meter. [4] The effect of such electric fields has been suggested as an explanation of cloud formation. The first time cloud ice forms around a clay particle, it requires a temperature of −10 ï¿½C, but subsequent freezing around the same clay particle requires a temperature of just −5 ï¿½C, suggesting some kind of "ice memory". [5] [ edit ] Surface tension Water drops are stable, due to the high surface tension of water. This can be seen when small quantities of water are put on a surface such as glass: the water stays together as drops. This property is important for life. For example, when water is carried through xylem up stems in plants the strong intermolecular attractions hold the water column together. Strong cohesive properties hold the water column together, and strong adhesive properties stick the water to the xylem, and prevent tension rupture caused by transpiration pull .

Other liquids with lower surface tension would have a higher tendency to "rip", forming vacuum or air pockets and rendering the xylem water transport inoperative. [ edit ] Electrical properties Pure water (de-ionised) is an excellent insulator as it does not conduct electricity at all. But no water is pure due to autoionisation at any temperature above absolute zero . Because water is such a good solvent, however, it almost always has some solute dissolved in it, most frequently a salt . If water has even a tiny amount of such impurities, then it can conduct electricity, as impurities such as salt separate into free ions in aqueous solution by which an electric current can flow. Water can be split into its constituent elements, hydrogen and oxygen, by passing a current through it. This process is called electrolysis . Water molecules naturally dissociate into H+ and OH− ions, which are pulled toward the cathode and anode , respectively. At the cathode, two H+ ions pick up electrons and form H2 gas. At the anode, four OH− ions combine and release O2 gas, molecular water, and four electrons. The gases produced bubble to the surface, where they can be collected.

It is known that the theoretical maximum electrical resistivity for water is approximately 182 kilohm -m2/m (or 18.2 MΩï¿½cm2/cm) at 25 ï¿½C. This figure agrees well with what is typically seen on reverse osmosis , ultrafiltered and deionized ultrapure water systems used for instance, in semiconductor manufacturing plants. A salt or acid contaminant level exceeding that of even 100 parts per trillion (ppt) in ultrapure water will begin to noticeably lower its resistivity level by up to several kilohm-square meters/meter (a change of several hundred nanosiemens per meter of conductance). [ edit ] Dipolar nature of water An important feature of water is its polar nature. The water molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge. A molecule with such a charge difference is called a dipole . The charge differences cause water molecules to be attracted to each other (the relatively positive areas being attracted to the relatively negative areas) and to other polar molecules.

This attraction is known as hydrogen bonding , and explains many of the properties of water. Certain molecules, such as carbon dioxide, also have a difference in electronegativity between the atoms but the difference is that the shape of carbon dioxide is symmetrically aligned which makes the different charges "take out one other". This phenomenon of water can be seen if you hold an electrical source near a thin stream of water falling vertically, causing the stream to bend towards the electrical source. Although hydrogen bonding is a relatively weak attraction compared to the covalent bonds within the water molecule itself, it is responsible for a number of water's physical properties. One such property is its relatively high melting and boiling point temperatures; more heat energy is required to break the hydrogen bonds between molecules. The similar compound hydrogen sulfide (H2S), which has much weaker hydrogen bonding, is a gas at room temperature even though it has twice the molecular mass of water. The extra bonding between water molecules also gives liquid water a large specific heat capacity . This high heat capacity makes water a good heat storage medium. Hydrogen bonding also gives water its unusual behavior when freezing.

When cooled to near freezing point, the presence of hydrogen bonds means that the molecules, as they rearrange to minimize their energy, form the hexagonal crystal structure of ice that is actually of lower density: hence the solid form, ice, will float in water. In other words, water expands as it freezes, whereas almost all other materials shrink on solidification. An interesting consequence of the solid having a lower density than the liquid is that ice will melt if sufficient pressure is applied. With increasing pressure the melting point temperature drops and when the melting point temperature is lower than the ambient temperature the ice begins to melt. A significant increase of pressure is required to lower the melting point temperature by very muchï¿½the pressure exerted by an ice skater on the ice would only reduce the melting point by something like 0.09 ï¿½C. [ edit ] Water as a solvent Water is also a good solvent due to its polarity . When an ionic or polar compound enters water, it is surrounded by water molecules ( Hydration ). The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute .

The partially negative dipole ends of the water are attracted to positively charged components of the solute, and vice versa for the positive dipole ends. In general, ionic and polar substances such as acids , alcohols , and salts are relatively soluble in water, and nonpolar substances such as fats and oils are not. Nonpolar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with nonpolar molecules. An example of an ionic solute is table salt ; the sodium chloride, NaCl, separates into Na+ cations and Cl- anions , each being surrounded by water molecules. The ions are then easily transported away from their crystalline lattice into solution. An example of a nonionic solute is table sugar . The water dipoles make hydrogen bonds with the polar regions of the sugar molecule (OH groups) and allow it to be carried away into solution. The solvent properties of water are vital in biology , because many biochemical reactions take place only within aqueous solutions (e.g., reactions in the cytoplasm and blood ).

[ edit ] Amphoteric nature of water Chemically, water is amphoteric ï¿½ i.e., it is able to act as either an acid or a base . Occasionally the term hydroxic acid is used when water acts as an acid in a chemical reaction. At a pH of 7 (neutral), the concentration of hydroxide ions (OH−) is equal to that of the hydronium (H3O+) or hydrogen (H+) ions. If the equilibrium is disturbed, the solution becomes acidic (higher concentration of hydronium ions) or basic (higher concentration of hydroxide ions). Water can act as either an acid or a base in reactions. According to the Brï¿½nsted-Lowry system, an acid is defined as a species which donates a proton (an H+ ion) in a reaction, and a base as one which receives a proton. When reacting with a stronger acid, water acts as a base; when reacting with a stronger base, it acts as an acid.

For instance, it receives an H+ ion from HCl in the equilibrium: HCl + H2O ⇌ H3O+ + Cl− Here water is acting as a base, by receiving an H+ ion. In the reaction with ammonia , NH3, water donates an H+ ion, and is thus acting as an acid: NH3 + H2O ⇌ NH4+ + OH− [ edit ] Acidity in nature In theory, pure water has a pH of 7 at 298 K. In practice, pure water is very difficult to produce. Water left exposed to air for any length of time will rapidly dissolve carbon dioxide , forming a dilute solution of carbonic acid , with a limiting pH of about 5.7. As cloud droplets form in the atmosphere and as raindrops fall through the air minor amounts of CO2 are absorbed and thus most rain is slightly acidic. If high amounts of nitrogen and sulfur oxides are present in the air, they too will dissolve into the cloud and rain drops producing more serious acid rain problems. [ edit ] Hydrogen bonding in water A water molecule can form a maximum of four hydrogen bonds because it can accept two and donate two hydrogens.

Other molecules like hydrogen fluoride , ammonia , methanol form hydrogen bonds but they do not show anomalous behaviour of thermodynamic , kinetic or structural properties like those observed in water. The answer to the apparent difference between water and other hydrogen bonding liquids lies in the fact that apart from water none of the hydrogen bonding molecules can form four hydrogen bonds either due to an inability to donate/accept hydrogens or due to steric effects in bulky residues. In water local tetrahedral order due to the four hydrogen bonds gives rise to an open structure and a 3-dimensional bonding network, which exists in contrast to the closely packed structures of simple liquids . There is a great similarity between water and silica in their anomalous behaviour, even though one (water) is a liquid which has a hydrogen bonding network while the other (silica) has a covalent network with a very high melting point. One reason that water is well suited, and chosen, by life-forms, is that it exhibits its unique properties over a temperature regime that suits diverse biological processes , including hydration . It is believed that hydrogen bond in water is largely due to electrostatic forces and some amount of covalency.

The partial covalent nature of hydrogen bond predicted by Linus Pauling in 1930s is yet be to proven unambiguously by experiments and theoretical calculations. [ edit ] Quantum properties of molecular water Although the molecular formula of water is generally considered to be a stable result in molecular thermodynamics, recent work, started in 1995 has shown that at certain scales, water may act more like H3/2O than H2O at the subatomic quantum level. [6] This result could have significant ramifications at the level of, for example, the hydrogen bond in biological , chemical and physical systems. The experiment shows that when neutrons and protons collide with water, they scatter in a way that indicates that they only are affected by a ratio of 1.5:1 of hydrogen to oxygen respectively. However, the time-scale of this response is only seen at the level of attoseconds (10-18 seconds), and so is only relevant in highly resolved kinetic and dynamical systems. [7] [8] [ edit ] History In 1724, Gabriel Fahrenheit initially defined a temperature scale in which 60 degrees was the boiling point of water.

[1] In 1742, Anders Celsius defined the Celsius temperature scale with the freezing point of water at 100 degrees and the boiling point at standard atmospheric pressure at 0 degrees. The scale was reversed in 1744. The first scientific decomposition of water into hydrogen and oxygen, by electrolysis , was done in 1800 by William Nicholson , an English chemist. [ edit ] Systematic naming The accepted IUPAC name of water is simply "water", although there are two other systematic names which can be used to describe the molecule. The simplest and best systematic name of water is hydrogen oxide. This is analogous to related compounds such as hydrogen peroxide , hydrogen sulfide , and deuterium oxide (heavy water). Another systematic name, oxidane, is accepted by IUPAC as a parent name for the systematic naming of oxygen-based substituent groups , [9] although even these commonly have other recommended names. For example, the name hydroxyl is recommended over oxidanyl for the ï¿½OH group.

The name oxane is explicitly mentioned by the IUPAC as being unsuitable for this purpose, since it is already the name of a cyclic ether also known as tetrahydropyran in the Hantzsch-Widman system ; similar compounds include dioxane and trioxane . [ edit ] Systematic nomenclature and humor Main article: dihydrogen monoxide hoax Chemists sometimes refer to water as dihydrogen monoxide or DHMO, an overly pedantic systematic covalent name of this molecule, especially in parodies of chemical research that call for this "lethal chemical" to be banned. In 2004, the town of Aliso Viejo, California nearly banned foam cups after learning that DHMO was used in their production. [10] In reality, a more realistic systematic name would be hydrogen oxide, since the "di-" and "mon-" prefixes are superfluous. Hydrogen sulfide , H2S, is never referred to as "dihydrogen monosulfide", and hydrogen peroxide , H2O2, is never called "dihydrogen dioxide".

Some overzealous material safety data sheets for water list the following: Caution: May cause drowning! The systematic acid name of water is hydroxic acid or hydroxilic acid. Likewise, the systematic alkali name of water is hydrogen hydroxideï¿½both acid and alkali names exist for water because it is able to react both as an acid or an alkali, depending on the strength of the acid or alkali it is reacted with ( amphoteric ). None of these names are used widely outside of DHMO sites.

Question: What is the only known substance that naturally exists on Earth in all three chemical states?
Answer: Water